The first well-documented crossover into the mirror universe, in 2267.

The mirror universe is a parallel universe where the patterns of events move in similar manners to that of the primary universe, but the intentions and characterizations are different, often showing "evil" characteristics in place of "good," and vice versa. Starfleet Intelligence, by 2267, referred to this reality as "Terran Empire parallel." The mirror universe shares a similar history to that of the primary universe, and many of the same people exist in this reality, but they and the galaxy they live in are skewed versions of their "normal" counterparts. (Star Trek Adventuresmodule: Core Rulebook)

Because of the development of the mirror universe in canon and non-canon works over the years, several alternate mirror universes have been established. The page approaches the mirror universe from its most recent and complete canon-version with compliant non-canon works, accepting variations of the mirror universe presented in other non-canon works as alternate mirror universes.

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It is not clear if the mirror universe diverged from the primary universe at some point or always existed in tandem, mirroring events and peoples. If the two universes did diverge it was at a point many hundreds of years ago. Phlox noted that the "great works" of literature in both universes were roughly the same except that the primary universe characters were "weak and compassionate," with the exception of that William Shakespeare and his counterpart. (ENTepisode: "In a Mirror, Darkly")

In the mirror universe of Dark Mirror, Earth's history took a darker turn starting with the end of Homer's Iliad, where Achilles kills King Priam, instead of showing one moment of humanity. Khan Noonien Singh and the other supermen won the Eugenics Wars, but then wiped each other out. The Vulcans were also militaristic, as there was no Surak.

On 5 April2063, first contact between Vulcans and Terrans took place much as it did in the primary universe. However, once the Vulcans landed, instead of welcoming them with open arms, Zefram Cochrane shot the first Vulcan to step onto Terran soil and the Terrans proceeded to raid the Vulcan ship, the T'Plana-Hath. The gun used by Cochrane later would come into the possession of CommanderJonathan Archer, who wondered what would have happened had Cochrane not "turned the tables on [the Vulcans'] invasion force." With advanced Vulcan technology now at their disposal, the Terrans were able to expand the Terran Empire and conquer other worlds, including Vulcan. (ENTepisode: "In a Mirror, Darkly")

It is not clear when the Empire began. Archer once stated that the Empire had existed for "centuries" as of 2155. Archer did not mention how many centuries, but by his statement, the Empire can be traced back to at least 1955, suggesting that it was a Terran political unit before it became an interstellar empire. The credits sequence for In a Mirror, Darkly used footage of battles going back at least to the "Age of Sail." It also appears that it is the empire that conflicts with the Xindi.

In the Mirror Universe Trilogy First Contact was the point of divergence with the primary universe; at this point, Zefram Cochrane warned the Vulcans about the Borg. Humans and Vulcans formed a militaristic alliance to wipe out the Borg, after which they became the Empire.

The Mirror Universe Saga had the Empire come about because the Romulans conquered Earth and enslaved humans for almost a decade. When Earth overthrew the Romulans, it embarked on a program of conquest itself, and became the Terran Empire.

Shortly thereafter, Spock rose to become leader of the Terran Empire, proposing a series of reforms designed to make the Empire more secure and less-dictatorial in nature. These included a significant disarmament program. Unfortunately, once these reforms were complete, the Empire was unable to defend itself against the equally aggressive and powerful forces surrounding it. The Klingon-Cardassian Alliance overran the Empire, conquering Earth and leaving Terrans and Vulcans enslaved. (DS9episode: "Crossover")

During this time, the forces of the mirror universe began implementing safeguards to prevent another crossover event with the primary universe. Transporter design was altered to prevent interdimensional travel, requiring the creation of devices specifically for that purpose, including the multidimensional transporter. In the event of another crossover, those involved would be killed to prevent further interference. (DS9episodes: "Crossover", "Through the Looking Glass")

The circumstances faced by the Terran race as depicted in the 2017 Mirror Broken comic book series differs substantially from those seen in the Star Trek: Mirror Universe prose works — specifically, the Terran Empire still exists in rump-form, with humanity having been driven all the way back to the Sol system by the Klingon-Cardassian Alliance by the 2360s or 2370s and militarily contained there, and with Mirror Spock's reforms having largely failed as policy. These circumstances seemingly somewhat conflict with the information given in the DS9 episode "Crossover," but in this series, the events seen onscreen in DS9 still evidently canonically take place.

Bajor, formally occupied by the Terrans, was freed by the Alliance and soon joined it. In orbit around the planet, the command post/ore-processing facility Terok Nor was constructed.

A year later, Smiley crossed over to the primary universe and impersonated his counterpart long enough to capture Benjamin Sisko. He took Sisko back to the mirror universe, where he convinced him to impersonate his mirror-self who had been killed in a skirmish with Alliance ships. The rebels needed the other Sisko to win over Jennifer Sisko, Sisko's wife, who was working as a scientist for the Alliance. Jennifer had been developing a transpectral sensor array, which would have allowed the Alliance to locate rebel hideouts in the Badlands. Fortunately for the rebels, Ben was able to convince Jennifer to defect to the movement. (DS9episode: "Through the Looking Glass")

In the Badlands, Chakotay, a former slave, commanded a ship in the rebellion attempting to evade pursuit. There, he encountered a strange ship which had been catapulted seventy thousand light-years across the galaxy. (VOYnovella: The Mirror-Scaled Serpent)

While in the Federation's universe, Smiley downloaded information from Deep Space 9's computers, including the plans for the USS Defiant. In 2372, the rebels constructed their own version of the Defiant, but had trouble getting it to function properly. Jennifer lured Ben Sisko back into the mirror universe to help. By this time the rebellion had grown in strength, culminating in the capture of Terok Nor, which became a rebel base of operations. (DS9episode: "Shattered Mirror")

In 2374, Intendant Kira sent a thief, Bareil Antos, to the primary universe to steal one of the Bajoran Orbs, believing it would permit Kira to unite Bajor under her rule. The attempt failed, and Bareil returned to the mirror universe without the orb. (DS9episode: "Resurrection")

She sent Ezri Tigan, a Trill mercenary and Kira's lover, to give Quark the ransom-demand. Quark and his brother Rom stole the cloaking device from Klingon General Martok's ship and delivered it to Ezri, but decided at the last minute that they couldn't trust her to keep her side of the deal, and accompanied her to the mirror universe. There, all three were captured by the Terran rebels, who planned to keep the cloaking device. Ezri's companion Brunt freed them, delivering them to Regent Worf.

Aboard Worf's flagship, Quark and Rom were imprisoned along with Zek upon discovering Kira's plan, only to be later rescued by Ezri as revenge against Kira for killing Brunt. Ezri ended up joining the rebel cause. The primary universe Ferengi were allowed to return to the primary universe for having aided the rebels' defeat of Worf, a major victory for the rebellion. (DS9episode: "The Emperor's New Cloak"; DS9novel: Saturn's Children)

In late 2376, Intendant Kira managed to use a form of mind-control to manipulate the primary-universe Jem'HadarTaran'atar into doing her bidding. She had him attack Kira Nerys before fleeing Deep Space Nine, and eventually beamed him into the mirror universe. Shortly afterwards, she was killed by Iliana Ghemor. (DS9novel: Warpath)

From 2376 into 2381, after the fall of Terok Nor to the Cardassians, the Terran Rebellion united with the Mackenzie Calhoun and the remnants of the Romulan Empire along with the forces of Memory Omega with Jean-Luc Picard as their leader to liberate Earth, where they then proceeded to liberate other core worlds of the Terran Empire. Instigating a war between the Klingons and the Cardassians, the Rebellion was able to sue for peace and create the Galactic Commonwealth, a representative republic that was in the process of rebuilding its member worlds. (ST - Mirror Universenovel: Rise Like Lions)

By the 25th century, the Terran Rebellion had succeeded in throwing off Alliance rule and a restored Terran Empire was once again a major power in the quadrant, possessing technology and weapons roughly on par with 25th-century Starfleet in the primary universe. Terran Empire ships and forces were known to operate in and around Cardassian territory, and even conduct missions to use the Bajoran wormhole to infiltrate the counterpart universe. (STOmission: "Crack in the Mirror")

The mirror universe was first established in the episode "Mirror, Mirror;" following this, there were no further canon visits until the Deep Space Nine episode "Crossover." DS9 continued to explore the mirror universe in several episodes, and some years later, the series Enterprise also explored the mirror universe in the, "In a Mirror, Darkly" two-parter. Between these various canon productions, the mirror universe was the basis of many novels and comics; however, as each TV series made its own mark on the continuity of the mirror universe, the histories and futures established for the universe in non-canon works began to contradict onscreen canon. Most recently, several mirror universe novels have been published in the DS9 relaunch series and the Star Trek: Mirror Universe series, which integrate new non-canon stories into the complete canon mirror universe.